Many kinds of thermal transfer printing methods have conventionally been known. Of these methods, public attention has been attracted by a sublimation type thermal transfer printing method in which an image can be formed on a image-receiving sheet for thermal transfer printing by heating, with the use of a heating means such as a thermal head or a laser, a thermal transfer sheet which had been prepared by forming a thermal transfer layer containing sublimative dyes on a support such as a polyester film, and has been placed on the image-receiving sheet for thermal transfer printing. Such a sublimation type thermal transfer printing method has been utilized as an information recording means in many kinds of fields of art.
According to such a sublimation type thermal transfer printing method, it is possible to form an image in various colors in an extremely short period of time, which bears comparison with a color photographic image having excellent reproducibility in neutral tints and toning property.
This method may provide an advantage that there can be obtained an image which is very clear and excellent in transparency, since according to this method, resin in the receptor layer is dyed by means of a dye to form the image. Therefore, this method has extensively been applied to the formation of a transparent projection sheet to be used for a projecting apparatus such as an overhead projector (hereinafter abbreviated to "OHP").
With respect to such use, a conventional image-receiving sheet for thermal transfer printing for the OHP has a receptor layer formed on one surface of a transparent substrate having a thickness of about 100 .mu.m and made of polyethylene terephthalate (hereinafter abbreviated to "PET"), and a back surface layer formed on the other surface thereof.
On the one surface of the substrate, there is provided with a receptor layer made of thermoplastic resin such as, for example, saturated polyester resin, vinyl chloride-vinyl acetate copolymer and polycarbonate resin in order to receive sublimative dyes transferred from the thermal transfer sheet and keep the formed image, and, if necessary, also provided with an intermediate layer.
There is provided, as an intermediate layer, a layer for imparting a cushioning property to an image-receiving sheet for thermal transfer printing, when a substrate made of PET having a high rigidity is for example used, or a layer for imparting an antistatic property thereto.
On the other surface, i.e., the back surface of the substrate, there is provided, if necessary, with a back surface layer formed by applying a composition prepared by adding an organic filler made of acrylic resin, fluoroplastics, polyamide resin or the like and/or an inorganic filler such as silica to a binder of acrylic resin or the like, in order to prevent the image-receiving sheet for thermal transfer printing from being curled up and improve an slipping property.
A so-called "standard type" image-receiving sheet for thermal transfer printing is used for projecting an image of the image-receiving sheet not by means of transmitted light but by means of reflected light. The standard type image-receiving sheet has substantially the same structure as that of the transmission type image-receiving sheet described above except that there may be used as an substrate an opaque, for example, white plastic sheet made of PET, foamed PET or the like; ordinary paper; synthetic paper; the combination of them adhered to each other or the like.
There has also been utilized for various uses a so-called "sticker-type" image-receiving sheet for thermal transfer printing in which a receptor layer was formed on the one surface of a substrate, and adhesive layer made of adhesive agent and a release paper were provided in this order on the other surface of the substrate. The sticker-type image-receiving sheet is used by forming an image on the receptor layer by means of a thermal transfer printing, peeling the release paper and sticking the image-receiving sheet on a desired object.
In the conventional image-receiving sheets for thermal transfer printing, various release agents are added to the receptor layers thereof, or a release layer is formed on the receptor layer. As a release agent, there has often been used silicone or a compound thereof such as silicone resin, and curable modified silicone has especially been used to improve releasability from the thermal transfer sheet.
The use of a certain kind of silicone has however caused a problem that the receptor layer was easily subject to flaws through friction between the receptor layer side surface of an image-receiving sheet and the back surface of the other image-receiving sheet and/or through friction between the surface of the receptor layer and parts of a printer, during the feeding of the image-receiving sheets in the printer, resulting in occurrence of so called abnormal transfer printing of causing the heat-fusion of the heat transfer sheet and the image-receiving sheet at portions having the above-mentioned flaws of the latter, the transfer of the entire layer of the dyeing layer, and the peeling of the receptor layer from the substrate. This means the poor abrasion resistance of the conventional image-receiving sheets for thermal transfer printing.
There has also been known a curing type release agent in which amino-modified silicone and epoxy-modified silicone are utilized. Such a release agent has however a problem that a long period of time is required for the curing thereof.
There has been carried out a practical step of adding a large amount of such a silicone compound to a receptor layer or increasing a thickness of a release layer, in order to achieve a sufficient releasing property with the use of the silicone compound. Such a practical step may cause occurrence of problems of degradation in dyeing affinity and decrease in depth of a formed image.
There has also been carried out a practical step of causing silicone oil having active hydrogen, such as hydroxyl group-modified silicone oil, carboxyl-modified silicone oil or amino-modified silicone oil to react on a curing agent such as isocyanate compounds or organic metallic compounds to cure such a kind of silicone oil, so as to achieve an abrasion resistance as well as a releasing property. Such a practical step may cause occurrence of a problem that the above-mentioned reaction requires a baking process which must be carried out at a high temperature for a long period of time, and an aging process which must be carried out for a long period of time after a drying process, thus requiring a long period of time for the reaction, resulting in poor productivity. A baking process which is carried out at a low temperature for a short period of time, may provide insufficient releasing property even when an aging process is carried out after the baking process. When an amount of added curing agent and/or catalyst is increased to an extent that a releasing property can be obtained, there are caused problems that the service life of a coating composition becomes very short, the coating composition easily tends to gel before a coating process, and an application property of the coating composition is deteriorated.